Two people have been taken to hospital and 33 others treated by paramedics after being taken ill at an industrial estate.
Paramedics and firefighters were alerted to the incident at Newlands Industrial Park in Somersham, Huntingdon, Cambs, at 9.40am yesterday.
The fire service said the alert could be related to crop spraying in a nearby field. Businesses on the site were advised to close windows and doors.
The East of England Ambulance Service deployed its hazardous area response team (HART) to the site. Spokesman Gary Sanderson said a total of 35 casualties were assessed by HART and land paramedics at the scene. Two men were taken to Hinchingbrooke Hospital in Huntingdon for further checks.
The spokesman said the three emergency services worked very closely together to treat the casualties.